Processes for stabilizing heavy metals (the metals in Groups IIA through VIA, including but not limited to lead, cadmium, arsenic, zinc and chromium) in a matrix to reduce leaching potential under induced or natural leaching conditions with treatment chemistries involving ferric iron are known. In typical land disposal environments, and particularly when the matrices treated according to the above processes are disposed beneath the surface, the acidity of the associated pore water can increase and can slowly neutralize the matrix alkalinity due, for example, to the presence of decaying vegetation in the vicinity of the disposed material or potential influx of groundwater. The increased acidity can initiate conditions for chemical and/or biological reduction of ferric iron (Fe3+) present in the waste to ferrous iron (Fe2+). If such a reduction occurs, heavy metals can be released in the groundwater because ferrous iron is not an effective immobilizing agent. As an example, arsenic has the potential to remobilize over time from material treated with ferric sulfate and magnesium oxide.
The materials treated according to prior processes disclose various chemistries for stabilizing heavy metal-contaminated materials in various matrices and leaching scenarios. Treated materials are then disposed in the non-saturated zone, generally in secure landfills equipped with engineered leachate control systems.
No prior process addresses long-term stability of the treated material, particularly when the treated waste is disposed in a saturated zone with potential for remobilization of the stabilized metals, particularly remobilization due to significant changes in the pore water chemistry over time initiated, for example, by ongoing contact of the disposed material with groundwater. Accordingly, existing stabilizing methods are inadequate for disposal of treated material in the saturated zone because of the potential for remobilization of the heavy metals. Processes that provide long-term stability of treated heavy metal-contaminated matrices disposed under saturated subsurface conditions are needed.
Stanforth (U.S. Pat. No. 5,037,479) discloses a method for treating solid hazardous waste containing unacceptable levels of leachable metals such as lead, cadmium, and zinc, which includes the steps of mixing the solid waste with at least two additives, a pH buffering agent, and an additional agent which is a salt or acid containing an anion that forms insoluble or non-leachable forms of the leachable metal, each agent being selected from a group of agents.
Stanforth et al. (U.S. Pat. No. 5,202,033) discloses a method for treating solid hazardous waste containing unacceptable levels of a leachable metal, such as lead, cadmium, arsenic, zinc, copper, and chromium, where the method includes the steps of mixing the solid waste in situ with a phosphate source or a carbonate source or ferrous sulfate. An additional pH controlling agent is optionally added under conditions that support reaction between the additive and pH controlling agent and the metals, to convert the metals to a relatively stable non-leachable form.
Hooykaas et al. (U.S. Pat. No. 5,430,235) discloses a process for solidifying an arsenic-contaminated matrix as a rock-hard product using high dosages of a clay material, an iron salt, a manganese salt, an oxidizer, and a hydraulic binder such as Portland cement. The process disclosed in U.S. Pat. No. 5,430,235 has several disadvantages. Because of the requirement for a hydraulic binder, the process includes a curing period of 7 days or longer. The process also results in significant bulking (volume increase) of the treated waste materials. If dosage levels are lower than those identified as preferred, it is difficult to achieve solidification.
Hooykaas et al. (U.S. Pat. No. 5,347,077) discloses a process for solidifying contaminated soil, sediment, or sludge that may contain arsenic by adding iron, manganese, aluminum salts, and Portland cement at dosages of 20 percent by weight and higher. Again, the process requires a curing period and has the additional disadvantage of high bulking after treatment. Hooykaas et al. use an oxidizing agent to oxidize organic matter, since it is difficult to solidify the waste matrix in the presence of organic matter.
U.S. Pat. No. 5,252,003 (McGahan) discloses a process for controlling arsenic leaching from waste materials by adding iron (III) ions and magnesium (II) ions, preferably in the form of iron (III) sulfate and magnesium oxide.
Hager (U.S. Pat. No. 4,723,992) discloses a process for fixing pentavalent arsenic in soil by adding metal salts or iron, aluminum, or chromium and a weak organic acid.
Falk (U.S. Pat. No. 5,130,051) discloses a process for encapsulating waste that contains toxic metals, including arsenic, by adding a mixture of alkaline silicate and magnesium oxide; and, optionally, borax, a concentrated acid, a reducing agent, and fly ash at high dosage rates.
U.S. Pat. No. 5,859,306 (Stanforth) discloses a process for arsenate stabilization using aluminum compounds and alkaline buffer.
U.S. Pat. No. 6,254,312 (Chowdhury et al.) discloses a process for stabilization of heavy metal-contaminated materials using pH control, ORP control, and adsorption-coprecipitation agents.
Treatment chemistries for stabilizing heavy metal-contaminated waste materials using CaCO3 added as limestone (U.S. Pat. No. 4,889,640) or formed in situ (U.S. Pat. No. 5,275,739) are also known, but these do not address the issue of long term storage or stability in a saturated zone.
Acidic iron sulfate pollution termed “acid mine drainage” (AMD), characterized by a pH below about 2.0 which has devastating effects on the environment, kills virtually all microbial and vegetative life it contacts and mobilizes various heavy metals such as lead, cadmium, mercury, copper, arsenic, chromium, selenium, antimony, zinc from the mine tailings of various mining operations such as coal processing, sulfide ore processing and the like, or from the surrounding soil matrix. Such mine tailings contain iron pyrite (FeS2) which, over time, reacts with air and water, is converted microbiologically into ferrous sulfate and sulfuric acid. The ferrous iron thus formed further converts to the ferric form and accelerates the pyrite oxidation, thereby creating more ferrous sulfate and sulfuric acid. The rate limiting step in production of AMD is the ferrous to ferric oxidation. Microbial oxidation by Thiobacillus ferrooxidans increases the ferrous oxidation rate by a few orders of magnitude.
U.S. Pat. No. 5,362,394 describes a process for controlling AMD using a permeable reactive barrier (PRB) containing organic carbon downstream of the waste impoundment to intercept the plume that contains ferrous sulfate and sulfuric acid. However, a downstream PRB treats only the symptoms of AMD but does not prevent the problem in the first instance. Moreover, known AMD control processes do not address long term permanency of the solution and can be cost prohibitive to implement.